Multiple function vidicon tube including a transmission grid

ABSTRACT

An improved vidicon tube is produced by adding a transmission grid to a standard vidicon tube. The improved tube serves as an image buffer which is compatible with broadband and narrowband transmission lines and can be written electrically at high or low data rates as well as optically. Information optically impressed on the photoconductor can be transferred to storage on the transmission grid. The improved vidicon, when built with a standard T.V. compatible photoconductor, is used as an intrusion alarm. When the improved vidicon is built with a storage photoconductor, information which is electrically written on the transmission grid is converted to conductivity storage in the photoconductor. Appropriate supporting apparatus and methods of operating this versatile tube are discussed.

Poley MULTIPLE FUNCTION VIDICON TUBE INCLUDING A TRANSMISSION GRID [75] Inventor: Niel Myron Poley, Kingston, N.Y.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: Dec. 28, 1971 [21] Appl. No.: 213,082

[52] U.S.CI ..3l5/1l,315/12,313/68A [51] Int. Cl. H0lj 31/48 [58] Field of Search 315/11, 12; 313/68 A [56] References Cited UNITED STATES PATENTS 3,670,198 6/1972 Lehovic et a1. 315/11 2,998,541 8/1961 Lempert 315/12 3,087,087 4/1963 McNanney 328/124 X 3,293,484 12/1966 Nakayama et a1... 315/11 3,404,308 10/1968 Burns 315/12 3,480,824 11/1968 Faberes et a1. 315/12 3,594,607 7/1971 Frankland i 315/12 3,633,064 1/1972 Herman 313/68 R X 3.336.585 8/1967 Macovski 340/258 B i "4 114 l 3 1 E 3 Primary Examiner-Leland A. Sebastian Assistant ExaminerP. A. Nelson Attorney, Agent, or Firm-Maurice I-l. Klitzman; John Wynn [5 7] ABSTRACT An improved vidicon tube is produced by adding a transmission grid to a standard vidicon tube. The improved tube serves as an image buffer which is compatible with broadband and narrowband transmission lines and can be written electrically at high or low data rates as well as optically. Information optically impressed on the photoconductor can be transferred to storage on the transmission grid. The improved vidicon, when built with a standard T.V. compatible photoconductor, is used as an intrusion alarm. When the improved vidicon is built with a storage photoconductor, information which is electrically written on the transmission grid is converted to conductivity storage in the photoconductor. Appropriate supporting apparatus and methods of operatingthis versatile tube are discussed.

4 Claims, 7 Drawing Figures 3,809,946 May 7, 1974 :ATENTEDMAY 111111 3.809.946

SHEEI 1 [If 4 ELECTRON (PRIOR A'RT) FOCUS GRID 1 1110001101101011 GUN 16 TRANSPARENT 12 FOCUS ELECTRODE 14 CONDUCTOR A 1s MODULATION Q 1 ig fi/ f 11 8 1 1 FACEPLATE I 1' 4/ 1 I VIDEO AMPLIFIER SCAN 24 SCAN GENERATOR PI 23 E L PHOTOCONDUCTOR TRANSMISSION 116 11(2FOCUSELECTRODE @1110 126 TRANSPARENT MODULATION DEFLEChON 7/ I i CONDUCTOR MEANS MEANS FOCUSGR'D 1;EA ss g 33 '1 FACEPLATE 11% 1 i 111 VIDEO DIELECTRIC COATING 12s" AMPLIFIER 0011011011111; MESH 121 A 125 SCAN INPUT BIAS 129 G SCAN 122 ENERATOR BIA SOURCE SHEEI 2 0F 4 FIG.3

ELECTRON ENERGY SECONDARY EMISSION RATIO I52 DIFFUSER SCAN MULTIPLE FUNCTION VIDICON TUBE INCLUDING A TRANSMISSION GRID BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the field of vidicon tubes and scan converter tubes.

2. Prior Art It is often necessary to convert information which is presented at one data rate to information at a second data rate for re-transmission. The need for this results from different pieces of electrical machinery operating at different data rates and different transmission lines being capable of transmitting information at different maximum rates.

Conversion of data from a first data rate to a second data rate is required for efficient use of transmission lines of varying bandwidths which form part of the same data path. A computer transmits and receives data at a very high rate over a broadband input/output channel. This information is often received or transmitted over narrowband telephone lines or other narrowband transmission systems which connect the computer to more remote units such as a data sensor or remote user terminals. In order to make the most efficient use of the computer, which is the most expensive unit, the information must be transmitted to or received from the computer at its data rate. However, since the information is transmitted or received over a narrowband transmission line which has a low data rate, the information cannot be transmitted or received at the computers data rate. Therefore, it becoms necessary to convert the information to be transmitted by the computer from the computers high data rate to the transmission lines low data rate and vice versa.

As our society has become more and more of an information processing society, efficient conversion between different data rates has become more and more important. For this reason, the prior art has developed methods of converting information from a first data to a second data rate. One of the ways of data conversion, which has become popular is the scan converting tube.

Prior art scan converter tubes can be divided into two groups. the first group consisting of those using two electron beams, one for writing and one for reading; and the second group consisting of those using a single beam for both writing and reading. In one prior art system of the first type a second scan converter tube with two electron beams uses the first electron beam, scanned at a first rate, to store the data received at the first data rate. Charges in a dielectric storage device are scanned at a second rate by a second electron beam to read the information at the second data rate for transmission. Both of these tubes are complicated and perform only a single function and therefore are expensive on a cost per function basis.

Two prior art scan converter tubes of the second type are the transmission grid scan converter tube and the storage photoconductor scan converter tube. In the transmission grid scan converter tube, information is written on a dielectric layer storage device by the electron beam which is scanned at a first rate. Information is read from the storage device by the same electron beam while swept at a second rate, and that portion of the beam passing through the storage device is collected and variations in the collected charge constitute the information at the second data rate. In the storage photoconductor scan converter tube, information is written on the storage photoconductor by sweeping the electron beam which is intensity modulated by the received data across the photoconductor at a first rate while the photoconductor is illuminated with diffuse light. Once the information has been written on the photoconductor, the light is turned off and the beam is continuously swept across the photoconductor (without modulation) to retain the memory of the photoconductor. Information is read at the second data rate by sweeping the electron beam across the photoconductor at a second rate while amplifying the current flowing through the photoconductor. The variations in the current to the photoconductor constitute the data at the second data rate. Both of these tubes have significant disadvantages as scan converter tubes. First, the transmission grid scan converter tube, although capable of almost indefinite storage (weeks or months) when the information is not read out, suffers a 50 percent loss in resolution in a short period of time (about 5 minutes) when read out continuously. A storage photoconductor scan converter tube can be read out continuously for up to an hour without major losses in resolution; however, the tube must be continuously read in order to retain the storage, thus limiting storage to that period of time. Further, the information must be writteninto the tube rapidly enough that the unmodulated beam returns to scan the first information written within the relaxation time of the tube which is generally on the order of a second. This drastically limits the use of this tube to convert information received at a low data rate over a narrow data rate line to a high data rate for transmission over a high data rate line.

Many remote computer terminals display information on cathode ray tubes as a means of communicating information to the user. Where the information is transmitted over a narrowband transmission line, the signal must be converted from a low transmission data rate to a high data rate for driving the display. The information must also be stored for use in a continuously refreshed display. The low data rate at which the information is received can easily be below the minimum rate usable with a storage photoconductor scan converter and, therefore, if such a converter is to be used, it requires complicated circuitry for writing the information. While a transmission grid scan converter can be written at the low data rate, its short storage time while being read to refresh the display makes it unacceptable in many situations. Thus, use of the prior art scan converter tubes for both scan conversion and storage is not practical because they lack facility to store the information at the rate at which it is received, while still providing repeated resolution refreshing readouts. This is because the prior art has failed to provide in a single tube the ability to convert data from very low data rates to the higher display data rates, while simultaneously storing the information for repeated high resolution readouts without extensive external circuitry.

Further, prior art optically sensitive intrusion alarm systems have been unduly expensive because they have required a magnetic or other external storage medium for storing the surveyed scene in its non-intruded form for comparison with the repetitively scanned scene to determine the presence of an intruder by the difference between the magnetically stored and optically received scenes. It is the necessity of having both an optical system and an external storage system which has made the accomplishment of the alarm function so expensive.

OBJECTS OF THE INVENTION A primary object of the invention is to convert information from a first data rate to a second data rate in an improved manner.

Another object of the invention is to convert in a simple economic way information between first and second data rates with repetitious high resolution readouts.

Another object of the invention is to convert with a single tube both information from a first low data rate to a second higher data rate and to store the information for multiple high resolution readout to the transmissions at the second data rate.

Another object is to simplify the circuitry for writing information into a storage photoconductor scan converter tube at low data rates.

Another object is to improve the versatility of a vidicon tube.

A further object of the invention is to combine the benefits of the transmission grid scan converter tube with the benefits of the storage photoconductor scan converter tube, while eliminating most of their disadvantages.

A still further object is to detect the presence of an intruder by comparing reference information with a current view of the protected premises in an improved way.

DEFINITIONS Throughout this patent application the following terms shall be defined as follows:

SCAN CONVERTER The term SCAN CONVERTER shall mean a system for converting information received at a first data rate into information for transmission at a second data rate wherein the system employs a scan converter tube.

VIDICON The term VIDICON shall mean a vidicon tube and may include both standard T.V. compatible vidicons and storage vidicons.

T.V. VIDICON The term T.V. VIDICON shall refer to vidicons of the type normally used for television broadcasting and other motion transmission and shall exclude storage vidicons.

STORAGE VIDICON The term STORAGE VIDICON shall mean a vidicon which has the ability to store and continue to transmit an image in the absence of light as a result of its mem ory of previous current and light conditions.

TRANSMISSION GRID The term TRANSMISSION GRID shall mean a grid which controls the proportion of an electron beam passing through the grid by means of the voltage level at which it is biased and in accordance with a pattern of localized electric charges distributed on the grid.

TRANSMISSION GRID VIDICON The term TRANSMISSION GRID VIDICON shall refer to all vidicons having a transmission grid in accordance with this invention and will include both T.V. vidicons and storage vidicons having transmission grids.

TRANSMISSION GRID T.V. VIDICON The term TRANSMISSION GRID T.V. VIDICON shall mean a TV. vidicon having a transmission grid in accordance with this invention.

TRANSMISSION GRID STORAGE VIDICON The term TRANSMISSION GRID STORAGE VID- ICON shall mean a storage vidicon having a transmission grid in accordance with this invention.

STORAGE OR PERSISTANT PHOTOCONDUCTOR The term PHOTOCONDUCTOR shall mean a photoconductor such as is used in a multi-readout storage vidicon.

SUMMARY OF THE INVENTION The invention achieves the above and other objects, features and advantages by adding a transmission grid to a vidicon tube between the electron gun and the photoconductive target. The transmission grid can be a conducting mesh having a dielectric coating on the gun side thereof. The dielectric coating has a high resistance and a secondary electron emission ratio greater than one for a selected range of incident electron energies. An electron beam is used to write information on the transmission grid in the form of electric charge distributions by using the secondary electron emission ratio of the dielectric. Information is written on the transmission grid at either a high or a low data rate, making the transmission grid storage system compatible with both broadband and narrowband transmission lines. Information is written onto the transmission grid by an electrical external input, or by conversion of information which is optically projected on the photoconductor surface for conversion to storage on the transmission grid.

Information may be written on a storage photoconductor in this tube by illuminating the photoconductor while sweeping the electron beam (modulated with the information) across the photoconductor and then extinguishing the illumination and sweeping the unmodulated beam across the photoconductor to refresh the storage. Alternatively, information may be transferred from storage on the transmission grid to storage in the photoconductor by illuminating the photoconductor and sweeping an unmodulated electron beam across the transmission grid and photoconductors. With the transmission grid properly biased, the transmission grid modulates the electron beam in accordance with the information stored on the transmission grid. The modulated beam writes the information on the storage photoconductor and, once the information has been written, the light is extinguished and the transmission grid is biased to a point where it no longer effects the intensity of the electron beam striking the photoconductor. The electron beam is continuously swept to retain the storage in the photoconductor.

Information is read out of the transmission grid for transmission by illuminating the photoconductor with a diffuse light to cause it to conduct and act as a collector electrode for the electron beam. The electron beam is swept across the transmission grid with the grid biased to modulate the beam. The current collected by the photoconductor is amplified for transmission. The information is present as variations in the current collected by the photoconductor. Information is read from the photoconductor by amplifying the current flowing through the photoconductor while it is swept by an unmodulated electron beam and no light is striking the photoconductor. Here again, the information is present as variations in the current collected from the photoconductor.

The transmission grid and storage photoconductor combine to provide the ability to write information into this tube at as slow a rate as may be desired, along with long term storage prior to reading that information from the tube (storage on the transmission grid). Once it is desired to read the information out of the tube, a high resolution readout is obtained. If continuous readouts are desired, the information stored on the transmission grid is transferred to the photoconductor which provides continuous readouts for up to an hour with high resolution. If single, widely spaced in time readouts are desired, the information is retained on the transmission grid and high resolution readouts are obtained for a very long period of time. Thus, this tube combines the advantages of both the transmission grid, scan converter tube and storage photoconductor scan converter tube.

The vacuum tube referred to herein as a transmission grid vidicon has nine distinct modes of operation:

1. writing on the transmission grid from an external electrical source;

2. writing on the storage photoconductor from an external electrical source;

3. writing on a storage photoconductor from an external optical source;

4. reading information stored on the transmission grid;

5. reading information stored in the storage photoconductor;

6. reading from a T.V. photoconductor while information from an external optical source is impressed on the photoconductor;

7. transferring information from the transmission grid to the storage photoconductor;

8. transferring information from the photoconductor to the transmission grid; and

9. reading the sum of the information stored on the transmission grid and the information present on the photoconductor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary diagramatic showing of prior art vidicon.

FIG. 2 is a fragmentary diagramatic showing of transmission grid vidicon in accordance with this invention.

FIG. 3 is a diagram illustrating the secondary electron emission ratio of the dielectric in the transmission grid as a function of the incident electron energies.

FIG. 4 illustrates electronic writing onto the transmission grid from an external data signal.

FIG. 5 illustrates transfer of information present in the photoconductor to storage on the transmission grid.

FIG. 6 illustrates a transmission grid vidicon being used as an intrusion alarm.

FIG. 7 illustrates transfer of information from the transmission grid to the photoconductor in a storage vidicon.

DETAILED DESCRIPTION Prior Art FIG. 1 is an illustration of a prior art vidicon for comparison with the improved vidicon of this invention to assist in explaining the relationship of this invention to the prior art. The prior art vidicon has an electron gun 10 including modulation means 13 and deflection means 15 for the electron beam, a focus electrode 12, a field mesh or focus grid 14, a transparent conductor 18 on the inside of a glass face plate 20, and a photoconductor 16 on the transparent conductor .18. For readout purposes, a video amplifier 23 is capacitively coupled to the transparent conductor 18 which is biased positive through a resistor 21 by bias source 22. A scan generator 24 is connected to the electron guns deflection means 15 for scanning the electron beam across the target.

DETAILED DESCRIPTION OF VIDICON FIG. 2 is an illustration of a transmission grid vidicon in accordance with this invention. Like the prior art, this vidicon has an electron gun including modulation means 113 and deflection means 115 for the electron beam, a focus electrode 112 including a screen grid 114, a transparent conductor 118 on the back of a glass face plate 120 and a photoconductor 116 on the transparent conductor 118. This tube also has provision for capacitively coupling a video amplifier 123 to the transparent conductor 118 which is again biased positive through a resistor 121 by bias source 122. The tube also has provision for a scan generator 124 to deflect the electron beam. In addition to the above elements which are present in the prior art vidicon, the vidicon of this invention includes a transmission grid 126 between the electron gun 110 and the photoconductor 116. The transmission grid. 126 is illustrated as a conducting mesh 127 having a dielectric coating 128 deposited on the gun side thereof. A grid bias input 129 is connected to the conductor 127 of the transmission grid 126. An input'terminal 111 is provided for supplying a data input for the modulation means in the electron gun. As with the standard vidicon, the tube elements are enclosed in an evacuated housing to eliminate the detrimental effects of gas in between the tube elements.

The video amplifier 123 is preferably a conventional video amplifier. The scan generator 124 is conventional except that it provides a variable scan rate so that the electron beam can be scanned across the target at two different rates necessary for converting data from a first data rate to a second data rate. If two predetermined, fixed data rates are to be used, two separate standard scan generators having the predetermined scan rates may be provided along with a means for selecting the scan generator appropriate for the data rate being used. Where many different rates are to be used, it is preferable to have one scan generator with a variable rate. Scan generator 124 must naturally be adapted for driving the type of deflection means provided in the electron gun of the tube. Magnetic deflection is the most common means of deflection for this type of tube, however, any deflection means which provides the necessary scanning may be used.

The transmission grid 126 is a well known element in charge storage tubes such as those used in PPI radar systems. A transmission grid stores information in the form of electric charge distributions on its dielectric. The charges on the transmission grid control the portion of the electron beam passing through the apertures in the grid to which they are adjacent. The transmission grid 126 may consist of an aperture conductor plate having a dielectric coating deposited thereon or may consist of a self-supporting apertured dielectric sheet having a conductor deposited on the side away from the electron gun. Other structures are also acceptable if they can be voltage biased, support localized electric charges and control the passage of an electron beam therethrough in accordance with the charges and the bias. A conducting mesh-like plate having a dielectric coating thereon is preferred. The dielectric coating preferably has a secondary electron emission ratio which is greater than one for a selected range of incident electron energies to allow charges stored on the transmission grid to be erased at will by charging the dielectric positive and then discharging it with a low energy electron beam. One such way is to use a photocathode which emits photoelectrons and provide a source of radiation for selectively illuminating the dielectric. Potassium chloride, calcium fluoride and magnesium oxide are all acceptable dielectric members having a secondary electron emission ratio greater than one. The RC time of the structure is determined by the thickness of the dielectric deposited on the grid, long RC times are achieved by using very high resistance dielectrics and by making the dielectric layers thick.

In FIG. 3, the secondary electron emission ratio of potassium chloride is shown and is typical of acceptable dielectrics for secondary emission systems. This information is plotted with the abscissa as the incident energy of electrons and the ordinate as the secondary electron emission ratio. The secondary electron emission ratio is a ratio of the number of secondary electrons emitted for each incident electron. Line 130 has been drawn through all points on the graph at which the secondary emission ratio is equal to one. Curve 132 represents the secondary emission ratio as a function of incident electron energy. Curve 132 intersects the secondary-emission-ratio-equals-one line 130 at two points, 134 and 136. The points 134 and 136 are known as the first and second cross-over points, respectively. For incident electron energies below that of the first cross-over point 134 or greater than that of the second cross-over point 136, the secondary electron emission ratio is less than one and fewer electrons are emitted then strike the dielectric, and an incident electron beam will build up a negative charge on the dielectric. At incident energies between the energies corresponding to the first and second cross-over points, 134 and 136, the secondary electron emission ratio is greater than one and more secondary electrons will be emitted than there are incident electrons and a positive charge will build up on the dielectric. The point 135 on curve 132 is the point where the secondary emission ratio is greatest.

In FIG. 4, another embodiment, a transmission grid vidicon, set up for writing information onto the transmission grid 126 from an external data source (not shown) is shown. A variable bias supply 140 is attached through bias modulator 141 to the bias input 129 of transmission grid 126. The bias modulator 141 is shown as a simple non-inverting transistor amplifier having a data input 142; however, any system which provides a DC level and the ability to modulate that level at the data rate is acceptable. The bias supply biases the transmission grid 126 to the appropriate voltages as will be explained hereinafter in the description of the operation of the tube. The data being received is connected to either the electron gun data input 111 or the bias modulator data input 142. During writing onto transmission grid 126 from an external data source the photoconductor 116 is illuminated by diffused light supplied by passing the light from a light source through a diffuser 152.

In another embodiment FIG. 5 illustrates the tube set up for converting information present in the photoconductor 116 to charge storage on transmission grid 126. Here the output of video amplifier 123 drives conversion logic 144 which in turn drives the transmission grid through data input 142 and bias modulator 141. Conversion logic 144 can be an inverting amplifier or can be more complicated if higher resolution is desired. As illustrated, the information on the photoconductor 1 16 can be supplied by lens focusing the desired scene or image 162 on the photoconductor 116. In a transmission grid T.V. vidion, the information must be projected on the photoconductor until all of the information has been written onto the transmission grid 126. However, in the improved tube having a storage photoconductor, the projection of the information on the photoconductor may cease as soon as the photoconductor has been optically written, provided that the scan rate during conversion from photoconductor to transmission grid storage is fast enough to maintain the conductivity of the storage photoconductor 116 until the conversion is complete.

In still another embodiment FIG. 6 illustrates a transmission grid vidicon set up as an intrusion alarm. An image of the non-intruded scene 162 is written on transmission grid 126 as is explained in the discussion of the operation of the tube. The scene under surveillance 162 is projected on the photoconductor 116 and a current monitor 146 is driven by the output of amplifier 123 or is connected to transparent conductor 1 18. The current monitor 146 monitors the amount of current passing through the transparent conductor 118. Should this current vary from its predetermined value an alarm output is produced. The current monitor 146 may be any of many well known circuits which produce an output upon deviation of an input voltage from a preset value by more than a predetermined limit. The requirements on this circuitry is further explored in connection with the description of the operation of the tube hereinafter in the discussion of the operation.

FIG. 7 illustrates another embodiment showing the transfer of information stored on transmission grid 126 to storage in the storage photoconductor 116. It is well known in the art that a persistant storage photoconductor requires continued sweeping of the photoconductor by the electron beam in order to refresh the conductivity memory of the photoconductor. Photoconductor 116 is illuminated with diffused light from light source 150 through diffuser 152. The illumination of the photoconductor lasts for one or two scans of the transmission grid 126 and photoconductor 116 by the electron beam. Thereafter, the illumination is turned off. During the period of illumination, the scanning of the electron beam determines the conductivity of the photoconductor as is further explained hereinafter in the discussion of the operation. After the illumination is removed, the conductivity of a given point in photoconductor 116 remains proportional to the number of electrons which strike that point on the photoconductor during the writing operation. The number of electrons which strike the photoconductor varies in accordance with the localized charge on transmission grid 126, the more positive the charge on the transmission grid, the greater number strike the photoconductor. Thus, the writing operation transfers the information stored on transmission grid 126 to storage photoconductor 116.

When the transmission grid is biased between the first and second cross-over points, the transmission grid serves as an additional focusing element and the vidocon operates as a standard separate mesh vidicon.

The resolution ofa transmission grid vidicon depends on the size of the scanning electron beam and on the size of the mesh used for the transmission grid, 1000 line mesh produces acceptable resolution.

OPERATION OF THE INVENTION The vacuum tube of this invention has several modes of operation. These modes of operation may be divided into three groups: writing information into the tube, reading information out of the tube and transferring information within the tube.

A. Writing Information Into The Tube l. Writing Data from an Electrical Source Onto the Transmission Grid: When information is to be written onto transmission grid 126 in response to an electrical input signal, the transparent conductor 118 is biased to a level and the photoconductor 116 is illuminated to cause it to conduct and to act as a collector electrode for the electron beam. Because of the positive bias on conductor I18 and the light on photoconductor 116, positive charges migrate to the back of photoconductor 116 where they are neutralized by beam electrons. As is illustrated in FIG. 4, it is preferable to use diffuse light to assure even featureless illumination. Transmission grid 126 is biased to a voltage below the first crossover voltage so that an impinging electron beam will build up a negative charge on the transmission grid. The electron beam is scanned across transmission grid 126 at a rate compatible with the data rate of the information to be written onto the grid. As the beam is being scanned, the number of charges deposited on transmission grid 126 is modulated in accordance with the data to be written on the grid. In order that the data subsequently read from the tube will have the same polarity as the input data, the more positive the voltage level of the input data, the more positive should be the charge on grid 126 after writing, provided the video amplifier is noninverting. The quantity of charge deposited on grid I26 can be controlled by modulating either the electron beam intensity or the bias supply to transmission grid 126. As will be explained below, the more positive the input data, the less should be the intensity of the electron beam if beam modulation is being used and the more positive the bias to grid 126 should be if transmission grid bias modulation is being used to control the writing. If beam intensity modulation is being used. grid 126 is biased to collect all of the electrons in the electron beam. This bias point would be preferably below the first cross-over voltage so that the grid will be charged negatively. Under these conditions the negative charge deposited on the grid will be proportional to the beam intensity so that to charge grid 126 relatively positive, the beam intensity should be reduced, while to charge grid 126 negative, the beam intensity should be increased. In the event that transmission grid 126 is biased between the first and second cross-over voltages, then the inverse relationships of beam intensity apply, since the more electrons which strike the dielectric, the more positive the charge on the dielectric becomes.

When transmission grid bias modulation is used to control the quantity of charge deposited on grid 126, and grid 126 is biased below the voltage corresponding to the maximum secondary emission ratio of the dielectric (point on curve 132 in FIG. 3), the bias should be driven positive to deposit a more positive charge on transmission grid 126, and more negative to deposit a more negative charge. If grid 126 is biased above the voltage corresponding to the maximum secondary emission ratio, then the above relationships are reversed. I

A special case of writing information on, the transmission grid is the erasure of information which is no longer needed. For erasure, the tube is set up as in FIG. 4, with transparent conductor 118 biased positive, the photoconductor illuminated with diffuse light and the transmission grid biased between the first and second crossover points 134 and 136 and scanned by the electron beam to charge the entire surface positive. The bias on grid 126 is then reduced to a voltage level well below the first cross-over point and the grid is scanned with a low energy electron beam so that the dielectric collects electrons to neutralize the positive charge and prepare the transmission grid for subsequent writing of other data. The low energy beam is used to prevent transmission grid 126 from accumulating a negative charge.

2. Writing Information Onto A Storage Photoconductor From An External Electrical Source:

The remembered conductivity ofa storage photoconductor in the absence of light depends both on the intensity of light incident upon it previously and on the level of current through it previously. Information can be electrically written onto a storage photoconductor by illuminating the photoconductor while scanning it with an electron beam of varying intensity. The light is then removed, and the scanning continued with an unmodulated beam. The current flowing out of the transparent conductor after the light is extinguished reflects the intensity levels of the electron beam during the period the light was on. However, in the absence'of scanning for a short duration, of the order of a few seconds, the memory of the photoconductor is erased and the photoconductor returns to its non-conducting state.

3. Writing Information Onto A Storage Photoconductor From An External Optical Source:

Information optically projected on the photoconductor can be stored by sweeping the photoconductor with a constant intensity electron beam. If optical input to the photoconductor is subsequently obstructed while the scanning of the constant intensity beam is continued, the photoconductor will retain the conductivity it had while the scene was projected on the photoconductor. Here again, failure to scan the photoconductor for a short period of time such as a second or two will result in erasure of the photoconductors memory. To

read the information stored in photoconductor l 16, the current collected by transparent conductor 1 18 is monitored by video amplifier 123 while the photoconductor is scanned with a constant intensity electron beam. The information stored in the photoconductor can be read out continuously for a long period of time (30 minutes to an hour) with only slight losses in resolution of the image. For this reason, the storage photoconductor readout is preferable to the transmission grid readout described above, when many copies of the information are to be read out continuously for a period of time.

B. Reading Information From The Tube 1. Reading Information From the Transmission Grid:

To read information from the transmission grid, the tube is set up as in writing on the transmission grid (FIG. 4) except that no data is applied to either the electron gun or bias modulator 141 and the bias on the grid may be different. Light source 150 again floods photoconductor 116 with diffuse light through diffusor 152 in order to cause the photoconductor to conduct and act as a collector electrode for the electron beam. Transparent conductor 118 is biased positive by source 122. The current flowing through transparent conductor 118 is amplified by the video amplifier I23. Variations in the output of the video amplifier constitute the data being readout.

The more positive the charge on the transmission grid, the more of the electron beam passes through the transmission grid, striking photoconductor 116 and flowing through transparent conductor 118, thus contributing to the current amplified by video amplifier 123. Since the more positive input voltages yield more positive charges on the transmission grid, the data read out of the scan converter at the output of video amplifier 123 is noninverted for a non-inverting amplifier. In the event the video amplifier is inverting, then the modulation criterion would be reversed in writing on grid 126 to prevent inversion of the data.

Repeated readouts of data from a transmission grid result in a fairly rapid loss of resolution in the image stored on the transmission grid. Thus, over a 5 minute span of continuous readouts, the image loses much of its resolution. This loss in resolution has several causes. First, electrons striking transmission grid 126 alter the quality of charge store at that position. Second, ions generated from residual gases or the surfaces within the tube may also reach and neutralize charges on grid 126. In the absence of an electron beam, the transmission grid can retain its charge distribution pattern (resolution) for days or weeks.

Readout from the transmission grid is very useful where only a small number of repetitous readouts is desired or where the readouts are widely separated in time. However, where larger numbers of readouts (copies) are desired, this transmission grid system can be inadequate because of its rapid loss of resolution under those conditions.

2. Reading Data From Storage in a Storage Photoconductor:

To read information stored in a storage photoconductor, the current flowing through the transparent conductor 118 as a result of the scanning of the electron beam to refresh the storage is amplified by video amplifier 123. The information stored in the photoconductor is present in the output of amplifier 123 as variations in the signal level.

3. Reading From a T.V. Photoconductor:

If information is to be read from a T.V. photoconductor, the information must be optically projected on the photoconductor and a positive bias must be applied to the transparent conductor 118. The current flowing through the transparent conductor is amplified by video amplifier 123 while the electron beam is scanned across the photoconductor. Since the conductivity of the photoconductor 116 varies in accordance with the quantity of light striking it, variations in the intensity of the light are converted to variations in the amplitude of the electrical signal at the output of video amplifier 123.

The third group of modes in which this tube may be operated can be referred to as the combined or transfer modes. In these modes, the two storage means in the tube cooperate simultaneously during the transfer of information.

C. TRANSFER OF INFORMATION WITHIN THE TUBE 1. Transfer of Information From the Transmission Grid to the Storage Photoconductor:

When it is desired to transfer information from the transmission grid to the storage photoconductor, the photoconductor is illuminated with a diffuse light, a positive bias is applied to the transparent conductor 118 and the transmission grid 126 is biased as for readout. The electron beam is scanned across transmission grid and photoconductor. The transmission grid modulates the electron beam as in readout from the transmission grid. The modulated electron beam strikes the photoconductor and establishes its conductivity in the same way as when the photoconductor is being written by an external electrical signal. As in writing from an external electrical signal, the light is turned off once the information has been written on the photoconductor and the photoconductor is scanned by an unmodulated electron beam. To present an unmodulated beam, the transmission grid is bias sufficiently positive that the entire electron beam passes through it unattenuated. At this bias level, the transmission grid serves as a focus electrode.

2. Transfer of Data From the Photoconductor to Transmission Grid Storage:

Information present in photoconductor 116 may be transferred to storage on the transmission grid 126 by the circuitry illustrated in FIG. 5. The information in the photoconductor is shown as a view of scene 162 focused on photoconductor 116 by lens but with a storage photoconductor, the information may be stored in the photoconductor instead. Information is transferred from the photoconductor to the grid by scanning the electron beam across the photoconductor (and grid) while monitoring the current flowing to transparent conductor 118 from bias source 122 through resistor 121. The monitoring is done by video amplifier 123 which is connected to drive conversion logic 144 which provides the drive signal for bias modulator 141. The transmission grid is biased in accordance with the current flowing through the photoconductor so that the transmission grid assumes a charge state corresponding to the information stored in the photoconductor. For this operation the conversion logic 144 can be a straight wire if the output level of the video amplifier is appropriate for driving the bias modulator 141. Otherwise, logic 144 can be either an amplifier or an attenuator. Circuit delays in photoconductor 116, transparent conductor 118, video amplifier 123, conversion logic 144 and transmission grid-bias modulator 141 cause the information stored on transmission grid 126 to be slightly skewed from the information on the photoconductor. The slower the scanning of the electron beam. the less the amount of skew because the beam moves a shorter distance during the delay time.

The skew between the information on photoconductor 116 and transmission grid 126 can be reduced by scanning the electron beam across the photoconductor to retrace its previous scan path in the reverse direction, while continuing to monitor the current flowing through the transparent conductor and varying the bias on the transmission grid in accordance with the current monitored to reduce any discrepencies between the information stored on the transmission grid and the infor mation contained in the photoconductor. This reverse scan serves as a form of feedback to adjust the charge stored on transmission grid 126 to correspond to the conductivity of photoconductor 116 and reduce the skew between the transmission grid image and the photoconductor image.

The conversion logic 144 is changed to an inverter having an appropriate gain or loss when it is desired to store on transmission grid 126, the inverse of the information present on photoconductor 116. Except for those areas where no light illuminates the photoconductor so that it is non-conducting, the level of charge on the transmission grid and the conductivity of the photoconductor can be balanced so that the current ouput from transparent conductor 118 is a constant level everywhere but in black spots. By alternating the direction of scan along the same scan path, skew and the variations in current it causes can be eliminated. Adjusting the current from transparent conductor 118 to a constant level establishes on transmission grid 126, a high resolution image of the information present in photoconductor 116.

All of the above methods of operation apply to a transmission grid vidicon whether it has a T.V. photoconductor or storage photoconductor.

3. Addition of the Information on the Transmission Grid and the Information on the Photoconductor:

The final mode of operation of this improved tube can be referred to as an addition mode. In this mode, information is stored on the transmission grid and is present on the photoconductor, and the transmission grid is biased to modulate the electron beam. The electron beam is modulated by the information stored in transmission grid 126. This electron beam is then col lected by the photoconductor 118 in accordance with the information present on the photoconductor. The current flowing through transparent conductor 118 is modulated by both the information present on the transmission grid and the information present in the photoconductor. Thus, in the sense, the information on the transmission grid is added" to the information on the photoconductor. It should be understood that this is not necessarily arithmetic addition, since it is possible for either the photoconductor or transmission grid to completely exclude the full current at one point, and because of non-linearities in their effects on the current.

This additive feature can be used in a number of ways. First, when the tube is operated as anormal vidicon for transmission purposes, portions of the image may be deleted from the transmission signal by charging the corresponding areas of the transmission grid negative and biasing the transmission so that negative charge includes the beam. With the transmission grid charged and biased, the areas of the image corresponding to the charged areas of the transmission grid are transmitted as though there were no light shining on those portions of the photoconductor. In effect, the charges on transmission grid 126 blank the scanning beam in those areas and serve as a memory for selective-mid-scan blanking of the electron beam. The blanking memory is useful for longer than an image on the transmission grid, when the tube is being used as a transmission grid scan converter because all of the charged areas of the grid are charged negative and are not discharged by the beam and the resolution does not need to be as good where the image is used forblanking, rather than as data. Second, the relative brightness of various portions of the image broadcast can be adjusted by charging transmission grid 126 to reflect part of the beam in those areas whose brightness is to be diminished, while allowing all of the beams to pass through the grid in those areas whose brightness is not to be decreased. Third, the tube can be used as an intrusion alarm.

4. Use ofa Transmission Grid Vidicon as an Intrusion Alarm:

To use a vidicon having a transmission grid and a T.V. photoconductor as an intrusion alarm, theinverse of the non-intruded scene being protected is stored on transmission grid 126 in the form of a charge distribution. This information may be supplied by an external electrical signal, but is preferably provided in the manner described above, wherein the information present on the photoconductor is stored in inverted form on the transmission grid to provide a constant current output level during scanning. To use the inverting method, the protected scene is projected into the photoconductor to provide the information to be stored on transmission grid 126. The information is then stored on the transmission grid. Once the non-intruded inverse of the protected scene. is stored on transmission grid 126, the protected scene 162 is focused on photoconductor 116 by lens as shown in FIG. 6 and the electron beam is scanned across transmission grid 126 and photoconductor 116 while the current collected by transparent conductor 118 is monitored. The monitoring circuit produces an alarm signal whenever the current being monitored indicates that the scene stored on the transmission grid differs from the information focused on the photoconductor. With the current set to a constant level, the monitoring circuit can be simply a level detecting circuit which produces an output whenever the monitored level deviates either positive or negative from the preset level. When such a monitoring circuit is used, the protected scene must be illuminated so that there are no black spots which result in a current lower than the established constant current. Alternatively, the current may be monitored by several circuits such that an alarm signal is given whenever the current level differs from the preestablished constant level and does not correspond to the current level associated with black areas.

Another alternative for current monitoring is to monitor the average current level and the occurrence of currents in excess of the established level. This system will detect the presence of an enlarged black area in a protected scene through a reduction in the average current level and will detect the presence of any new nonblack object. Thus, even an intruder 164 familiar with the fact that the system does not accurately respond to black objects would be unable to intrude undetected by wearing an all black outfit because he would reduce the average current. The use of a suit corresponding to the established current level is impossible because in passing across a black area would cause a decrease in current to below the constant level. Both of these occurrences would be detected and result in an alarm being activated.

The intrusion alarm system is equally useful as a comparison means for comparing information projected on the photoconductor with information stored on the transmission grid in the above described inverse form.

Once an intrusion has been detected and an alarm sounded, the transmission grid vidicon can be converted to standard vidicon operation to transmit a continuous view of the intruded area.

Although, a transmission grid T.Vv vidicon is preferred for use in an intrusion alarm because of its response characteristics, a transmission grid storage vidicon can be used with degraded results.

5. Scan Conversion Using a Transmission Grid Storage Vidicon:

There are several modes of operation in which a transmission grid storage vidicon may be used as a scan converter. Incoming information may be written directly on photoconductor 116, if the data rate of the information is high enough to allow the electron beam to write the information on the entire photoconductor and still return to the first information written on the photoconductor in time to refresh the memory of the photoconductor. If the readout data rate is high enough to allow the entire photoconductor to be read while still refreshing its memory, then the information may be read out in the same fashion as in a normal storage vidicon. If the data rate is too slow for refreshing by the scanning beam, then the scanning beam must be alternated between reading out part of the information at the data rate and refreshing all the information at a faster sweep rate, or a flood gun must be used for refreshing the memory while the scan electron beam reads the desired information. if photoconductor readout is desired. If photoconductor readout is not required, the tube can be used as a transmission grid scan converter.

Where the incoming data is received at too slow a rate to allow the entire photoconductor to be written in time for the beam to return to the first written portion in time to refresh that memory, then the incoming information is preferably written on the transmission grid. Once all the data has been written on the transmission grid, that information may be transferred to the storage photoconductor. Thereafter, readout may proceed in the fashion of the previous paragraph.

Where only a few repetitious readouts of the information are desired, the information may be written on the transmission grid and read off the transmission grid, without any loss in resolution. However, if many repetitious readouts are desired, photoconductor storage is preferable because of its longer readout time without loss of resolution.

While the invention has been discussed in terms of the preferred embodiments thereof, the preferred methods of operating those embodiments and variations therein, it will be clear to those skilled in the art that many changes in the structure, supporting circuitry and methods of operation, both as to form and details may be made without departing from the spirit and scope of the invention.

I claim:

1. An improved multiple function vidicon tube comprising:

an evacuated housing;

an electron gun at a first side of said housing for providing a direct beam of electrons within said evacuated housing;

an optical face plate forming the second side of said housing at which said beam of electrons is directed;

a target comprising an optically transparent conductor inside said housing in back of said face plate and a photoconductor layer contiguous with said transparent conductor; and V a transmission grid located between said electron gun and said photoconductor for controlling said electron beam striking said photoconductor in accordance with the charge on said transmission grid.

2. The apparatus of claim 1 wherein said transmission grid is comprised of:

an apertured conductor; and

a high resistance dielectric layer on the side of said apertured conductor which faces said electron gun, said dielectric layer having a secondary electron emission ratio which is greater than one for incident electrons within a selected energy range to enable said electron beam to charge said dielectric layer positive or negative in order to store information on said dielectric layer.

3. The apparatus of claim 2 wherein said photoconductor is a storage photoconductor to provide another storage means within said storage vidicon tube.

4. A scan converter system utilizing an improved multiple function vidicon tube comprising:

an evacuated housing;

an electron gun at a first side of said housing for providing a directed beam of electrons within said evacuated housing;

an optical face plate forming the second side of said housing at which said beam of electrons is directed;

a target comprising, an optically transparent conductor inside said housing in back of said face plate, and a photoconductor layer contiguous with said transparent conductor;

a transmission grid forming a first storage means located between said electron gun and said photoconductor for controlling said electron beam striking said photoconductor in accordance with the charge on said transmission grid, said transmission grid comprising an apertured conductor and a high resistance dielectric layer on the side of said apertured conductor which faces said electron gun, said dielectric layer having a secondary electron emission ratio which is greater than one for incident electrons within a selected energy range to enable said dielectric layer to be charged positive when incident electrons within the selected energy range strike said dielectric layer, and having a secondary electron emission ratio which is less than one for incident electrons of other energies to enable said electron beam to charge said dielectric layer negative when the electrons of the other energies strike said dielectric layer so as to selectively control the portion of the electron beam striking selected areas of said photoconductor, said photoconductor being a storage photoconductor to provide a second storage means within said storage vidicon tube;

a diffused light source for illuminating said photoconductor to cause it to conduct;

scanning means for scanning said electron beam across said transmission grid and photoconductor;

a video output amplifier connected to said transparent conductor contiguous with said photoconductor for amplifying data therefrom;

variable bias means connected to said transmission grid for biasing said transmission grid to control the passage of the beam of electrons and the rate of emission of secondary electrons;

a bias modulator for modulating said variable bias means for changing the secondary emission ratio of said dielectric layer in accordance with data to be stored on said transmission grid;

a control means connected to said video output amplifier for driving said bias modulator in accordance with the output of said video amplifier in order to transfer data from said photoconductor to said transmission grid.

[UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 r 8 I 946 Dated May 7 197,4

Inventor-(s) Nell yron Poley It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title Page, [75]Niel Myron Poley" should read Neil Myron Poley--.

Signed and sealed this 8th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. Attesting Officer C. MARSHALL DANN Commissioner of Patents FORM PO-105O (10-69) v USCOMM DC 503764369 w u.s, sovannmzm mm'rmc. ornca: was 0-3s6-334 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,809,946 Dated May 7, 1974 lhventofls) Nell Myron P oley It is certified that ertor appears in the above-identified, patent V and that said Letters Patent are hereby corrected as shown below:

Title Page, [75]"Niel Myron Poley" should read Neil Myron Poley--.

Signed and sealed this 8th day of October 197 (SEAL) Attest: I

MCCOY M; GIBSON JR. 4 c. MARSHALL DANN Atte s ting Officer Commissionerof Patents Foam Po-mso (10459) V USCOMWM G -fa A LLS, GOVERNMENT PRINTING OFFICE. I969 0-356-334 

1. An improved multiple function vidicon tube comprising: an evacuated housing; an electron gun at a first side of said housing for providing a direct beam of electrons within said evacuated housing; an optical face plate forming the second side of said housing at which said beam of electrons is directed; a target comprising an optically transparent conductor inside said housing in back of said face plate and a photoconductor layer contiguous with said transparent conductor; and a transmission grid located between said electron gun and said photoconductor for controlling said electron beam striking said photoconductor in accordance with the charge on said transmission grid.
 2. The apparatus of claim 1 wherein said transmission grid is comprised of: an apertured conductor; and a high resistance dielectric layer on the side of said apertured conductor which faces said electron gun, said dielectric layer having a secondary electron emission ratio which is greater than one for incident electrons within a selected energy range to enable said electron beam to charge said dielectric layer positive or negative in order to store information on said dielectric layer.
 3. The apparatus of claim 2 wherein said photoconductor is a storage photoconductor to provide another storage means within said storage vidicon tube.
 4. A scan converter system utilizing an improved multiple function vidicon tube comprising: an evacuated housing; an electron gun at a first side of said housing for providing a directed beam of electrons within said evacuated housing; an optical face plate forming the second side of said housing at which said beam of electrons is directed; a target comprising, an optically transparent conductor inside said housing in back of said face plate, and a photoconductor layer contiguous with said transparent conductor; a transmission grid forming a first storage means located between said electron gun and said photoconductor for controlling said electron beam striking said photoconductor in accordance with the charge on said transmission grid, said transmission grid comprising an apertured conductor and a high resistance dielectric layer on the side of said apertured conductor which faces said electron gun, said dielectric layer having a secondary electron emission ratio which is greater than one for incident electrons within a selected energy range to enable said dielectric layer to be charged positive when incident electrons within the selected energy range strike said dielectric layer, and having a secondary electron emission ratio which is less than one for incident electrons of other energies to enable said electron beam to charge said dielectric layer negative when the electrons of the other energies strike said dielectric layer so as to selectively control the portion of the electron beam striking selected areas of said photoconductor, said photoconductor being a storage photoconductor to provide a second storage means within said storage vidicon tube; a diffused light source for illuminating said photoconductor to cause it to conduct; scanning means for scanning said electron beam across said transmission grid and photoconductor; a video output amplifier connected to said transparent conductor contiguous with said photoconductor for amplifying data therefrom; variable bias means connected to said transmission grid for biasing said transmission grid to control the passage of the beam of electrons and the rate of emission of secondary electrons; a bias modulator for modulating said variable bias means for changing the secondary emission ratio of said dielectric layer in accordance with data to be stored on said transmission grid; a control means connected to said video output amplifier for driving said bias modulator in accordance with the output of said video amplifier in order to transfer data from said photoconductor to said transmission grid. 